DEVELOPMENT OF TELEPHONE SERVICE 499
lines ; then he installed a home-made switching device in his office, and for a signaling device employed a single-stroke bell operating on a gravity-battery current. Then he devised a method of preventing eavesdropping that in May, 1878, was declared ' a great improvement on any other telephone system now in use.' So satisfactory did these instruments prove that other lines were similarly equipped and soon a mutual telephone exchange system was in full operation, that later on formed the nucleus of the commercial exchange opened in Bridgeport by Mr. Doolittle in 1878.
In the fall of 1877 he planned and built a private telephone ex- change system for the Ansonia Brass and Copper Company, of An- sonia, that, while in no sense a commercial exchange system, was indi- rectly of incalculable benefit to the growth and prosperity of the entire commercial telephone industry. For in planning this system Mr. Doolittle decided to use circuits of copper instead of iron, and, after many experiments, produced a hard drawn copper wire of his own adaptation, the drawing of which he supervised in the mills of that company.
Prior to that time it had been found impossible to use copper wire on pole line circuits, as its extreme ductility proved a source of con- tinued elongation in all spans of any length and where the strain was constant. By the Doolittle process the tensile strength of the wire was greatly increased, its elongation reduced to about one per cent., and there was no appreciable change in its conductivity. Yet it re- quired ten years of costly experience with iron wire circuits before the telephone interests fully comprehended the inestimable value of this improvement. In September, 1880, at a conference of telephone men, the representative of a very large wire-drawing mill stated that " copper wire has ceased to be discussed for telephone line use. It is too soft and elongates too readily under exposure. The suitable wire must be tough pure iron, well manipulated to secure flexibility and toughness."
Eeferring to this pioneer work, Mr. Doolittle wrote:
Hard drawn copper was the result of an adaptation rather than a discovery, although many of its valuable properties were not appreciated until after it had been in service several years. Prior to its introduction for aerial conductors there was very little, if any, call for the hard product. Copper wire was usually annealed after drawing and sold in that form. Copper alloyed with other metal was, and is now, used in the manufacture of hard or spring wire. It is the com- mon knowledge of all who are familiar with the manipulation of copper that the process of drawing it into wire serves to harden the surface. Thus it will be seen that the experiments which resulted in the so-called hard drawn copper wire were based upon a well-known principle, although the application of this principle had never been made use of for the final product. The writer was familiar with this phenomenon of the hardening of drawn copper at the time he entered the field of electricity; therefore, when it was disclosed to him that copper was not only one of the best conductors of electricity, but was the cheapest in conductivity, or per mile ohm, it was only left for him to determine whether or not this hardening process could be made available in order that copper wire